The oldest mentions found of sword swallowers are of performers in India at least several millennia BC.
Sword swallowing was performed along with several other notoriously dangerous looking acts, like coal walking and snake charming.
Around the year zero this performance art spread to Greece and Rome.
Soon after, it also spread to Asia and Europe and the rest we know.

Just like in the present times, these performances were typically done as a proof of profound spirituality or extreme self discipline.
The spirituality aspect might be scientifically debatable, but the knowledge of how to control the self to perform these skills certainly is an interesting finesse in itself.

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The sword used is generally not sharpened.
Round edges make the sword go down easier.
It is still dangerous because a sword is hard and pointy
and our flesh is soft and squishy.
Mastering the gag reflexes and holding perfectly still in a pose that guides the sword perfectly straight down is the part that requires a lot of training and self control.
Gag reflexes are a natural defence system against eating stuff we shouldn’t. Blades are certainly amongst those forbidden foods.
Another hard part is never to panic or you will move in any way that makes the sword pierce you anyway.
Even skilled swallowers can still get hurt performing this trick. Because even the best practice does not prevent the unexpected to happen in this act of extreme precision where there is not even a little bit of room for any accidents.
Before attempting this trick that walks a fine line between super safe and utterly lethal, get professional help.
From a veteran sword swallower, from a psychiatrist or even better: both!

Sword swallowing is a dangerous skill.
But long and careful practice enables people to actually perform it kinda safely, or so they say, before a captive audience, that still can’t get enough of watching other people do dangerous stuff.

• http://www.swordswallow.org/ssad.php